Small Molecule Receptors for Membrane Lipids Project Summary Cell membranes consist of a variety of lipids. It is clear that they do not simply serve as a physical barrier of a cell. Instead the composition and distribution of membrane lipids have significant ramifications for physiology and disease. For example, bacterial and mammalian cells display dramatically different compositions in their plasma membranes, with bacterial membrane harboring a large percentage of the negatively charged phosphatidylglycerol (PG). Another well known example is phosphatidylserine (PS), which in healthy mammalian cells is exclusively confined to the inner leaflet of the plasma membrane. Externalization of PS to the cell surface is a hallmark event of apoptosis, a primary mechanism of cell death. While we begin to appreciate the significance of lipids, much remains to be uncovered on the intricate details of their function in a variety of biological processes. Toward this end, small molecules that recognize a specific lipid are highly desirable for profiling the spatiotemporal distribution of lipids. We hypothesize that small cyclic peptides could serve as an effective and versatile scaffold for designing low molecular weight receptors for membrane lipids. Our preliminary studies demonstrated the feasibility of this approach: cyclic peptides mimicking the natural protein lactadherin (cLac) display protein-like specificity for PS and effectively label apoptotic cells. Aim 1 of this submission seeks to further develop the cLac design as PS receptors. We will explore the potential of pre- organization, polyvalency and covalent chemistry to improve the binding affinity and specificity of cLac towards PS-presenting membranes. Using the cLac peptide as a blueprint, we will design solid phase supported libraries of cyclic peptides. Screening of the peptide libraries will enable discovery of small molecule receptors for a variety of lipids. In aim 2 of this proposal, we will expand the cyclic peptide design to develop ligands for the bacterial lipid PG. Specifically, we will use the cyclic scaffold to display chemical functionalities that bind PG head groups through covalent chemistry. The capability of targeting PG will enable novel strategies for the design of membrane-lytic antibiotics.